This invention relates to the liquefying of natural gas. In one of its aspects this invention relates to the liquefying of natural gas containing at least 1.5 percent nitrogen. In another of its aspects this invention relates to improving the efficiency of a system for liquefying natural gas. In yet another of its aspects this invention relates to the removal of nitrogen from a cascade cycle in the liquefying of natural gas. In still another aspect of the invention it relates to stabilizing the nitrogen recycle in a cascade refrigeration system for liquefying natural gas containing nitrogen. The presence of nitrogen in natural gas feedstocks in quantities of about 1.5 percent or more presents problems in the liquefaction of the gas. Liquefaction systems depending on a cascade system cooling which recycle flashed vapors through a recompression system with return of the recompressed gases into the feed gas build up recycle volume of nitrogen. This problem is partially alleviated by purging some of the flash vapors from the system, generally for use as fuel, thereby removing some of the nitrogen that would otherwise be recycled. When the amount of nitrogen in the incoming gas is 1.5 percent of that gas, either the amount of vapors purged becomes excessive or the increased volume of nitrogen recycled causes the recompression system to become increasingly inefficient. The stabilization of the amount of nitrogen within the recycle system then becomes imperative.
It is therefore an object of this invention to provide a method for stabilizing the amount of nitrogen recycle is a cascade liquefaction system for natural gas. It is another object of this invention to provide apparatus and method for removing nitrogen from a cascade system for liquefying natural gas utilizing thermal capacity within the system. It is still another object of the invention to provide method and apparatus for reducing the power consumption of the recompression system within a cascade system for liquefying natural gas (LNG).
Other aspects, objects, and the various advantages of this invention will become apparent upon study of this specification, the drawings, and the appended claims.